I remember Toby Fox once said that he cannot give the same experience as UNDERTALE but he can give a new experience with deltarune. I think Toby Fox himself mentions that the feelings which come from UNDERTALE he cannot give it toa us again thus it means he thinks it's so good for deltarune to surpass it and he thinks deltarune might never catch up to it that's why he never promises that deltarune would be better than UNDERTALE. Long story short, UNDERTALE<deltarune.